This invention relates to load indicating members and, more particularly, to load indicating members, such as fasteners, having ultrasonic transducers.
In many operations, it is desirable to determine the amount of longitudinal load experienced by a longitudinally stressed member. This information is particularly useful when the longitudinally stressed member is a fastener since the measurement of longitudinal load provides a verification of the existence of a proper joint.
Ultrasonic load measurement is a precise measurement technique for determining load in bolted joints. Pulse-echo techniques with removable ultrasonic transducers have been used in laboratories and for quality control for over thirty years. Historically, however, the practical difficulties in achieving reliable acoustic coupling and in incorporating transducers in tool drives have prevented this technique from becoming a general assembly tightening strategy.
The above coupling difficulties were overcome with permanently attached transducers. U.S. Pat. No. 4,846,001 (issued to Kibblewhite) teaches the use of a thin piezoelectric polymer film which is permanently, mechanically, and acoustically coupled to an end surface of a member and used to determine the length, tensile load, stress or other tensile load-dependent characteristic of the member by ultrasonic techniques. Although the invention represented a significant advance over the prior state of the art in terms of performance, ease of manufacture, and manufacturing cost, there are disadvantages with a transducer of this construction. These disadvantages relate to environmental performance, in particular the maximum temperature limitations of the polymer material which restricts its application, and the possibility of the transducer, fixed to the fastener with adhesive, coming loose and causing an obstruction in, or damage to, a critical assembly.
U.S. Pat. No. 5,131,276, issued to Kibblewhite and assigned to Ultrafast, Inc., teaches a load-indicating member having an ultrasonic transducer, including an acousto-electric film, grown directly on the fastener surface (i.e., a piezoelectric thin-film). By growing the acousto-electric film directly on the fastener, the film is mechanically, electrically, and acoustically interconnected to the surface. Permanent ultrasonic transducers not only allow the precise pulse-echo load measurement technique to be used in production assembly but also significantly improve accuracies by eliminating errors that result from axial and radial movement of the removable transducer relative to the bolt and from variations in the coupling media.
All the above-mentioned ultrasonic methods of determining load in a load indicating member require a zero-load measurement in addition to the measurement taken under the desired loaded condition in order to determine the absolute load in the member. Furthermore, all use a direct or indirect measurement of the out-and-return time-of-flight of a longitudinal ultrasonic wave. Holt, U.S. Pat. No. 4,602,511, teaches of a method which uses the times-of-flight of both longitudinal and transverse waves to determine the stress in a member without taking a zero-load measurement. This is desirable in the measurement of tensile load in previously installed fasteners, for example.
The use of transverse ultrasonic waves, however, requires both a transducer capable of generating transverse waves and an acoustic coupling media capable of transmitting transverse waves into the member. Special acoustic couplants are required with temporarily attached transducers, since transverse waves cannot generally be transmitted through liquids. Although adhesives can transmit transverse ultrasonic waves, generation of transverse waves using the polymer film transducers disclosed by Kibblewhite in U.S. Pat. No. 4,846,001 has not been demonstrated. Only the permanent ultrasonic transducer technology disclosed by Kibblewhite in U.S. Pat. No. 5,131,276 has demonstrated a practical method of making load measurements in fasteners without first taking a zero-load measurement using the method based on measurements of both longitudinal and transverse waves. However, accuracies of only ±15% are typically achievable with this method due to production variations in the material and geometry of the fasteners.
The above-mentioned ultrasonic load measurement methods using longitudinal waves alone are capable of precise measurements when a zero-load measurement is made prior to tightening, with typical accuracies of ±3% documented. Because of the variation in initial lengths of fasteners manufactured using production methods, measurement of installed load at a later time is only possible with ultrasonic load measurements using longitudinal waves alone by recording the zero-load length measurement. Ultrasonic load measurement instruments have the ability to store and retrieve zero-load measurements for later inspection of load.
A means of identifying each fastener for storing and retrieving zero-load length measurements with removable transducer ultrasonic load measurement instrumentation is disclosed by Shigemi et al. in Japanese Patent Application Publication No. 10-086074. Shigemi discloses a method of applying an identifying mark, such as a bar code, on the periphery of the head of the fastener. The bar code is read by an optical bar code reader and the zero-load ultrasonic length measurement is stored in memory in a control device corresponding to the identifying mark on the fastener. When reading the fastener load at a later date, the identifying mark is first read to retrieve the zero-load length measurement. Zero-load bolt length is the only ultrasonic parameter associated with a specific bolt disclosed by Shigemi. Also, the bar code disclosed by Shigemi is used for identification purposes only and contains no encoded ultrasonic measurement information. While this invention is suitable for a single instrument at a single location, storing and retrieving a zero-load length alone is inadequate in ensuring reliable precise measurement with all fastener types, with different instruments or with different ultrasonic transducers, such as at multiple service locations, for example.
A difficulty in making reliable fastener load measurements with ultrasonic pulse-echo instrumentation arises from the uncertainty in consistently identifying the same echo cycle to which time-of-flight measurements are made. Considerable distortion of the echo waveform can occur, especially with fasteners with large length-to-diameter ratios, primarily due to fastener geometry and stress distribution variations. Vecchio et al., in U.S. Pat. No. 6,009,380, disclose a multi-frequency excitation and detection method to improve the reliability of detecting the correct echo cycle when making ultrasonic time-of-flight measurements for determining fastener load. The method stores characteristics of a typical echo waveform as a reference for a particular fastener type. However, variations in echo waveforms from fasteners of the same type can be sufficiently large to prevent this method, using a single reference for a particular fastener type, from working reliably for all fasteners. Consequently, fasteners which deviate significantly from the reference waveform characteristics are unsuitable for reliable inspection with this method and must be screened out in production.